The seed layer is a very thin layer of metal, e.g., Pd, on a polymeric substrate, capable of acting as a surface catalyst for the plating of Cu thereon. Thus, a very critical aspect of additive plating is the seed layer. If the seed layer is too thin, it will not be catalytic for the electroless deposition of copper, while if it is too thick, it will be resistant to removal at the end of the fabrication process and may lead to resist adhesive failure during the fabrication process. The adsorption of polyelectrolytes, especially cationic polyelectrolytes, onto electronic package substrate surfaces, enhances the adsorption of colloidal Pd/Sn particles thereto, which, in turn catalyzes the electroless deposition of copper circuitization onto electronic package surfaces. In a typical process sequence an organic polymer having pendant ionizable groups, i.e., a polyelectrolyte, is adsorbed from solution onto an electronic package substrate, followed by adsorption of colloidal Pd/Sn as a seed layer. A photoresist is applied atop the seed layer and photolithographically imaged to uncover the regions of the seed layer to be circuitized. Copper is electrolessly applied to the photolithographically uncovered seed layer. The uncovered Pd reduces Cu in the electroless plating bath to form the circuitization. The remaining photoresist is removed, i.e., stripped, and the remaining uncovered seed and polyelectrolyte are removed, leaving only residual seed and polyelectrolyte underlying the Cu circuitization.
A key requirement of the process is to achieve the appropriate Pd/Sn seed catalyst loading. Insufficient Pd catalyst will result in voids in the copper deposit, creating open circuits. Too much catalyst can cause both adhesive failure and lateral conduction. Adhesive failure permits the electroless plating solution to leak beneath the photoresist and deposit copper between circuit elements, causing short circuits.
Deposition of the Pd/Sn seed or catalyst layer depends critically on the adsorbed polyelectrolyte. The Pd/Sn colloidal particles do not appreciably adsorb on the electronic package substrate surface, that is, on the untreated electronic package surface. Thus, a need exists to controllably enhance the adsorption of colloidal Pd/Sn particles onto the electronic package substrate.